U.S. patent number 4,760,745 [Application Number 06/938,404] was granted by the patent office on 1988-08-02 for magnetoelastic torque transducer.
This patent grant is currently assigned to Mag Dev Inc.. Invention is credited to Ivan J. Garshelis.
United States Patent |
4,760,745 |
Garshelis |
August 2, 1988 |
**Please see images for:
( Certificate of Correction ) ** |
Magnetoelastic torque transducer
Abstract
A magnetoelastic torque transducer for providing an electrical
signal indicative of the torque applied to a member, the member
including ferromagnetic, magnetostrictive means affixed to,
associated with or forming a part of the surface of the torqued
member for altering in magnetic permeability in response to the
application of torque to the member. The ferromagnetic,
magnetostrictive means is advantageously formed of nickel maraging
steel, desirably 18% Ni maraging steel. Preferably, the transducer
comprises a pair of axially spaced-apart annular bands defined
within a region of the ferromagnetic, magnetostrictive means, the
bands being endowed with residual stress created, respectively
symmetrical right and left hand helically directed magnetic
anisotropy of sufficiently large magnitude that the contribution to
total magnetic anisotropy of any random anisotropy in the member is
negligible. In a particularly preferred aspect of the invention,
each said band has at least one circumferential region which is
free of residually unstressed areas over at least 50% of its
circumferential length.
Inventors: |
Garshelis; Ivan J. (Cheshire,
MA) |
Assignee: |
Mag Dev Inc. (Pittsfield,
MA)
|
Family
ID: |
25471381 |
Appl.
No.: |
06/938,404 |
Filed: |
December 5, 1986 |
Current U.S.
Class: |
73/862.333;
73/862.334; 73/862.336 |
Current CPC
Class: |
G01L
3/102 (20130101) |
Current International
Class: |
G01L
3/10 (20060101); G01L 003/10 () |
Field of
Search: |
;73/862.36,779,DIG.2 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
|
|
|
|
|
|
|
169326 |
|
Jan 1945 |
|
JP |
|
58-9034 |
|
Jan 1983 |
|
JP |
|
0079240 |
|
May 1985 |
|
JP |
|
0192233 |
|
Sep 1985 |
|
JP |
|
0274420 |
|
Jun 1970 |
|
SU |
|
667836 |
|
Jun 1979 |
|
SU |
|
838448 |
|
Jun 1981 |
|
SU |
|
Other References
Beth and Meeks, "Magnetic Measurement of Torque in a Rotating
Shaft", The Review of Scientific Instruments, vol. 25, No. 6, Jun.
1954. .
Harada et al., "A New Torque Transducer Using Stress Sensitive
Amorphous Ribbons", IEEE Trans. on Mag., MAG-18, No. 6, 1767-9,
(1982). .
Sasada et al, "Torque Transducers with Stress-Sensitive Amorphous
Ribbons of Chevron-Pattern", IEEE Trans. on Mag., MAG-20, No. 5,
951-53 (1984). .
Mohri, "Review on Recent Advances in the Field of Amorphous-Metal
Sensors and Transducers", IEEE Trans. on Mag., MAG-20, No. 5,
942-47 (1984). .
Yamasaki et al., "Torque Sensors Using Wire Explosion
Magnetostrictive Alloy Layers", IEEE Trans. on Mag., MAG-22, No. 5,
403-405 (1986). .
Sasada et al., "Noncontact Torque Sensors Using Magnetic Heads and
a magnetostrictive Layer on the Shaft Surface-Application of Plasma
Jet Spraying Process", IEEE Trans. on Mag., MAG-22, No. 5, 406-408
(1986). .
Sasada et al., "Noncontact Torque Sensor", presented 11/19/85 at
11th IEEE Industrial Electronics Society Conference. .
Angeid, "Noncontacting Torquemeters Utilizing Magneto-Elastic
Properties of Steel Shafts", ASME 69-GT-64 (1968). .
Sasada et al., "A New Method of Assembling a Torque Transducer by
the Use of Bilayer-Structure Amorphous Ribbons", IEEE Trans. on
Mag., MAG-19, No. 5, 2148-2150 (1983). .
Winterhoff et al., "Non-Contact Torque Measurements Using Eddy
Current Sensors", Technisches Messen (Germany), 50, No. 12, 461-466
(1983). .
Scoppe et al., "Practical Application of Magnetostriction to a High
Speed Torquemeter", 12 pages, presented Jun. 4-6, 1985, Thirteenth
Transducer Workshop,-Vehicular Instrumentation/Transducer
Committee, Telemetry Group, Range Commanders Council. .
Garshelis, "A Study of the Inverse Wiedemann Effect on Circular
Remanence", IEEE Trans. on Mag., MAG-10, No. 2, 344-358 (1974).
.
Garshelis, "The Wiedemann Effects and Applications", 38 pages,
presented Jun. 5-6, 1975 at Applied Magnetics Workshop, IEEE
Magnetics Society, Marquette University. .
Sasada et al., "Noncontact Torque Sensor Using Magnetic Heads and a
Magnetostrictive Layer on the Shaft Surface", Abstract (1986).
.
Hall et al., "The Metallurgy, Behavior, and Application of the
18-Percent Nickel Maraging Steels", NASA SP-5051, pp. 1-3, 5,
12-14, 23-41, 89-108, (1968). .
"18% Nickel Maraging Steels", The International Nickel Company,
Inc., pp. 4-30, (1964). .
VASCOMAX 200-250-300-350, "Application data for 18% Nickel Maraging
Steels", Teleydyne Vasco (1968)..
|
Primary Examiner: Ruehl; Charles A.
Attorney, Agent or Firm: Sixbey, Friedman, Leedom &
Ferguson
Claims
I claim:
1. A magnetoelastic torque transducer for providing an electrical
signal indicative of the torque applied to a member, said member
having a ferromagnetic and magnetostrictive region, said tranducer
comprising:
a pair of axially spaced-apart annular bands defined within said
region, said bands having respectively symmetrical right and left
hand helically directed residual stress-created magnetic anisotropy
of sufficiently large magnitude compared with the random magnetic
anisotropy in said member that the contribution to total magnetic
anisotropy of any random is negligible, each said band having at
least one circumferential region which is free of residually
unstressed areas over at least 50% of its circumferential
length;
means for applying a cyclically time varying magnetic field to said
bands;
means for sensing the change in permeability of said bands caused
by said applied torque; and,
means for converting said sensed change in permeability to an
electrical signal indicative of the magnitude of the torque applied
to said member.
2. A magnetoelastic torque transducer, as claimed in claim 1,
wherein each said band has at least one circumferential region
which is free of residually unstressed areas over at least 80% of
its circumferential length.
3. A magnetoelastic torque transducer, as claimed in claim 1,
wherein each said band has at least one continuous circumferential
region which is free of residually unstressed areas.
4. A magnetoelastic torque transducer, as claimed in claim 1,
wherein said region comprises the surface of said member.
5. A magnetoelastic torque transducer, as claimed in claim 1,
wherein said region is rigidly affixed to the surface of said
member.
6. A magnetoelastic torque transducer, as claimed in claim 1,
wherein the magnetic easy axes in said bands are oriented,
respectively, at angles of .+-.20.degree.-60.degree. to the axis of
said member.
7. A magnetoelastic torque transducer, as claimed in claims 1, 4,
5, 2, 3 or 6, wherein said region is formed of nickel maraging
steel.
8. A magnetoelastic torque transducer, as claimed in claim 7,
wherein said region is formed of 18% Ni maraging steel.
9. In a magnetoelastic torque transducer for providing an
electrical signal indicative of the torque applied to a member
including ferromagnetic, magnetostrictive means associated with
said member for altering in magnetic permeability in response to
the application of torque to said member, means for applying a
magnetic field to said ferromagnetic magnetostrictive means, means
for sensing the change in permeability caused to said applied
torque, and means for converting said sensed change in permeability
to an electrical signal indicative of the magnitude of the torque
applied to said member, the improvement comprising forming said
ferromagnetic, magnetostrictive means from nickel maraging
steel.
10. A magnetoelastic torque transducer, as claimed in claim 9,
wherein said ferromagnetic, magnetostrictive means comprises the
surface of said member.
11. A magnetoelastic torque transducer, as claimed in claim 9,
wherein said ferromagnetic, magnetostrictive means is rigidly
affixed to the surface of said member.
12. A magnetoelastic torque transducer, as claimed in claims 9, 10
or 11, wherein said ferromagnetic, magnetostrictive means is formed
of 18% Ni maraging steel.
13. A magnetoelastic torque transducer, as claimed in claims 9, 10
or 11, wherein at least a portion of said ferromagnetic,
magnetostrictive means is endowed with helically directed residual
stress created magnetic anisotropy, at least one circumferential
region of said portion being free of residually unstressed areas
over at least 50% of its circumferential length, said applying
means applying said magnetic field to said endowed portion and to
an area of said member not so endowed, said sensing means sensing
the permeability difference between said portion and said area
resulting from the application of torque to said member, said
converting means converting said sensed permeability difference to
an electrical signal indicative of the magnitude of the applied
torque.
14. A magnetoelastic torque transducer, as claimed in claim 13,
wherein said circumferential region is free of residually
unstressed areas over at least 80% of its circumferential
length.
15. A magnetoelastic torque transducer, as claimed in claim 13,
wherein said portion has at least one continuous circumferential
region which is free of residually unstressed areas.
16. A magnetoelastic torque transducer, as claimed in claims 9, 10
or 11, wherein said ferromagnetic, magnetostrictive means includes
a pair of axially spaced-apart annular bands defined therewithin,
said bands having respectively symmetrical right and left hand
helically directed residual stress created magnetic anisotropy,
each said band having at least one circumferential region which is
free of residually unstressed areas over at least 50% of its
circumferential length, said applying means applying said magnetic
field to said bands, said sensing means sensing the change in
permeability of said bands caused by said applied torque.
17. A magnetoelastic torque transducer, as claimed in claim 16,
wherein each said band has at least one circumferential region
which is free of residually unstressed areas over at least 80% of
its circumferential length.
18. A magnetoelastic torque transducer, as claimed in claim 16,
wherein each said band has at least one continuous circumferential
region which is free of residually unstressed areas.
19. A magnetoelastic torque transducer, as claimed in claim 16,
wherein the magnetic easy axes in said bands are oriented,
respectively, at angles of .+-.20.degree.-60.degree. to the axis of
said member.
20. A magnetoelastic torque transducer for providing an electrical
signal indicative of the torque applied to a member, said member
having a ferromagnetic and magnetostrictive region formed of nickel
maraging steel, said transducer comprising:
a pair of axially spaced-apart annular bands defined within said
region, said bands having respectively symmetrical right and left
hand helically directed residual stress-created magnetic
anisotropy, each said band having at least one circumferential
region which is free of residually unstressed areas over at least
50% of its circumferential length;
means for applying a cyclically time varying magnetic field to said
bands;
means for sensing the change in permeability of said bands caused
by said applied torque; and,
means for converting said sensed change in permeability to an
electrical signal indicative of the magnitude of the torque applied
to said member.
21. A magnetoelastic torque transducer, as claimed in claim 20,
wherein each said band has at least one circumferential region
which is free of residually unstressed areas over at least 80% of
its circumferential length.
22. A magnetoelastic torque transducer, as claimed in claim 20,
wherein each said band has at least one continuous circumferential
region which is free of residually unstressed areas.
23. A magnetoelastic torque transducer, as claimed in claims 20, 21
or 22, wherein said region comprises the surface of said
member.
24. A magnetoelastic torque transducer, as claimed in claims 20, 21
or 22, wherein said region is rigidly affixed to the surface of
said member.
25. A magnetoelastic torque transducer, as claimed in claims 20, 21
or 22, wherein said region is formed of 18% Ni maraging steel.
26. An internal combustion engine having a torque-carrying output
member, said member including a magnetoelastic torque transducer as
claimed in claims 1, 9 or 20.
27. Power transmission means having a torque-carrying output
member, said member including a magnetoelastic torque transducer as
claimed in claims 1, 9 or 20.
28. Fluid turbine means having a torque-carrying output member,
said member including a magnetoelastic torque transducer as claimed
in claims 1, 9 or 20.
29. A weighing system including torque-carrying means, said means
including a magnetoelastic torque transducer as claimed in claims
1, 9 or 20.
30. A machine tool including means for causing relative rotation
between a tool and a workpiece, said machine tool having a
torque-carrying member, said member including a magnetoelastic
torque transducer as claimed in claims 1, 9 or 20.
31. A robotic device comprising mechanical means for performing
work pursuant to pre-programmed or real time control instructions,
said device having a torque-carrying member, said member including
a magnetoelastic torque transducer as claimed in claims 1, 9 or
20.
32. A vehicular steering system having torque-carrying shaft means,
said means including a magnetoelastic torque transducer as claimed
in claims 1, 9 or 20.
33. A force measuring system including means for converting a
sensed force to torque, a torque transducer connected for sensing
said torque, said torque transducer comprising a magnetoelastic
torque transducer as claimed in claims 1, 9 or 20.
34. In a method of sensing the torque applied to a member having
ferromagnetic, magnetostrictive means associated therewith which
includes the steps of applying a magnetic field to said
ferromagnetic magnetostrictive means, sensing the change in
permeability caused by said applied torque and converting said
sensed change in permeability to an electrical signal indicative of
the magnitude of the applied torque, the improvement comprising
forming said ferromagnetic, magnetostrictive means form nickel
maraging steel.
35. A method, as claimed in claim 34, wherein said nickel maraging
steel is 18% Ni maraging steel.
36. A method, as claimed in claim 34, wherein said ferromagnetic,
magnetostrictive means is endowed with a pair of axially
spaced-apart annular bands having respectively symmetrical right
and left hand helically directed residual stress-created magnetic
anisotropy, the permeability difference between the bands is
sensed, and said sensed permeability difference is converted to an
electrical signal indicative of the magnitude of the applied
torque.
37. In a method of sensing the torque applied to a member having a
ferromagnetic and magnetostrictive region including the steps of
endowing at least a portion of said region with helically directed
magnetic anisotropy, applying a cyclically time varying magnetic
field to said portions and to an area of said member not so
endowed, and sensing the permeability difference between said
portion and said area resulting from the application of torque to
said member, the difference being indicative of the magnitude of
the applied torque, the improvement comprising forming said region
from nickel maraging steel.
38. In a method of sensing the torque applied to a member having a
ferromagnetic and magnetostrictive region, including the steps of
endowing a pair of axially spaced-apart annular bands within said
region with respectively symmetrical right and left hand helically
directed magnetic anisotropy, applying a cyclically time varying
magnetic field to said bands, and sensing the permeability
difference between said bands resulting from the application of
torque to said member, the difference being indicative of the
magnitude of the applied torque, the improvement comprising:
defining said bands at the surface of said member and endowing said
bands with magnetic anisotropy of sufficiently large magnitude
compared with the random magnetic anisotropy in said member that
the contribution to total magnetic anisotropy of any random
anisotropy is negligible by instilling a residual stress
distribution in each band which is sufficiently extensive that at
least one circumferential region within each band is free of
residually unstressed areas over at least 50% of its
circumferential length.
39. A method, as claimed in claim 38, wherein said instilled
residual stress distribution is sufficiently extensive that said
region is free of residually unstressed areas over at least 80% of
its circumferential length.
40. A method, as claimed in claim 38, wherein said instilled
residual stress distribution is sufficiently extensive that each
said band has at least one continuous circumferential region which
is free of residually unstressed areas.
41. A method, as claimed in claims 38, 39 or 40, wherein said
region is formed of nickel maraging steel.
42. In a method of sensing the torque applied to a member having a
ferromagnetic and magnetostrictive region, including the steps of
endowing a pair of axially spaced-apart annular bands within said
region with respectively symmetrical right and left hand helically
directed magnetic anisotropy, applying an alternating magnetic
field to said bands, and sensing the permeability difference
between said bands resulting from the application of torque to said
member, the difference being indicative of the magnitude of the
applied torque, the improvement comprising:
forming said region of nickel maraging steel, defining said bands
at the surface of said member and endowing said bands with magnetic
anisotropy by instilling a residual stress distribution in each
band which is sufficiently extensive that at least one
circumferential region within each band is free of residually
unstressed areas over at least 50% of its circumferential length.
Description
TECHNICAL FIELD
The present invention relates to torque sensors and, more
particularly, to non-contacting magnetoelastic torque transducers
for providing a measure of the torque applied to a rotary
shaft.
BACKGROUND ART
In the control of systems having rotating drive shafts, it is
generally recognized that torque is a fundamental parameter of
interest. Therefore, the sensing and measurement of torque in an
accurate, reliable and inexpensive manner has been a primary
objective of workers for several decades. Although great strides
have been made, there remains a compelling need for inexpensive
torque sensing devices which are capable of continuous torque
measurements over extended periods of time despite severe
environments.
All magnetoelastic torque transducers have two features in
common--(1) a torqued member which is ferromagnetic and
magnetostrictive, the former to ensure the existence of magnetic
domains and the latter to allow the orientation of the
magnetization within each domain to be altered by the stress
associated with applied torque; and (2) a means, most usually but
not necessarily electromagnetic means, for sensing variations from
the untorqued distribution of domain orientations. The differences
among the various existing or proposed magnetoelastic torque
transducers lie in the detailed variations of these common
features.
It is well known that the permeability of magnetic materials
changes due to applied stress. When a torsional stress is applied
to a cylindrical shaft of magnetostrictive material, each element
in the shaft is subjected to a shearing stress. This shearing
stress may be expressed in terms of a tensile stress and an equal
and perpendicular compressive stress with the magnitude of each
stress being directly proportional to the distance between the
shaft axis and the element. The directions of maximum tension and
compression occur along tangents to 45.degree. left-handed and
45.degree. right-handed helices about the axis of the shaft. The
effect of the torque is to increase the magnetic permeability in
directions parallel to one of the helices and, correspondingly, to
decrease the magnetic permeability in directions parallel to the
other of the helices. In their article "Magnetic Measurements of
Torque in a Rotating Shaft", The Review of Scientific Instruments,
Vol. 25, No. 6, June, 1954, Beth and Meeks suggest that in order to
use permeability change as a measure of the applied torque, one
should monitor permeability along the principal stress directions
and pass the magnetic flux through the shaft near its surface. This
is because the stress is greater, the further the element is from
the shaft axis and it is along the principal stress directions that
the maximum permeability change is expected. To accomplish this,
Beth and Meeks used a yoke carrying a driving coil for producing an
alternating flux in the shaft and pickup coils on each of several
branches to detect the permeability changes caused by the applied
torque in flux paths lying in or near the principal stress
directions in the shaft. When the shaft is subjected to a torque,
the mechanical stresses attributable to torque resolve into
mutually perpendicular compressive and tensile stresses which cause
the permeability in the shaft to increase in the direction of one
stress and decrease in the direction of the other. As a result, the
voltage induced in the pickup or measuring coils increases or
decreases. The difference in magnitude of the induced voltages is
proportional to the torsional stress applied to the shaft. A
similar approach was taken in U.S. Pat. No. 3,011,340--Dahle. The
principal shortcoming in these type devices is the need to
accomplish permeability sensing along the principal stress
directions with its attendant disadvantages, such as its
sensitivity to variations in radial distance from the shaft,
magnetic inhomogeneity around the shaft circumference and
noncompensatable dependence on shaft speed. As a result, devices
such as these have only found applications on large diameter
shafts, i.e., 6-inches and larger, but have not been found to be
adaptable to smaller shafts where the vast majority of applications
exist.
It was felt by some that devices such as were taught in Beth and
Meeks and U.S. Pat. No. 3,011,340--Dahle, wherein the rotating
shaft itself acted as the magnetic element in the transducer, had
significant drawbacks in practical application. This is because the
materials and metallurgical processing which may have been used to
impart the desired mechanical properties to the shaft for its
desired field of use will, in most cases, not be optimum or even
desirable for the magnetic qualities required in a magnetoelastic
torque sensor. The random anisotropy in a shaft created during its
manufacture, due to internal stresses and/or resulting from regions
of differing crystal orientation will cause localized variations in
the magnetic permeability of the shaft which will distort the
desired correlation between voltage sensed and applied torque. The
solution, according to U.S. Pat. No. 3,340,729--Scoppe is to
rigidly affix, as by welding, a magnetic sleeve to the
load-carrying shaft so that a torsional strain proportional to the
torsional load is imparted to the sleeve. The measuring device
employed now senses permeability changes in the rotating sleeve
rather than in the rotating shaft. This permits, according to
Scoppe, a material to be selected for the shaft which optimizes the
mechanical and strength properties required for the shaft while a
different material may be selected for the sleeve which optimizes
its magnetic properties. As with prior art devices, the Scoppe
torquemeter utilized a primary winding for generating a magnetic
flux and two secondary windings, one oriented in the tension
direction and the other in the compression direction. Although
obviating at least some of the materials problems presented by
Dahle, the use of a rigidly affixed sleeve creates other, equally
perplexing problems. For example, the task of fabricating and
attaching the sleeve is a formidable one and even when the
attachment means is welding, which eliminates the bond strength
problem, there remains the very significant problem that the
coefficient of thermal expansion of the steel shaft is different
(in some cases up to as much as 50% greater) than the corresponding
coefficient of any magnetic material selected for the sleeve. A
high temperature affixing process, such as welding, followed by
cooling establishes stresses in the magnetic material which alters
the resultant magnetic anisotropy in an uncontrolled manner.
Moreover, annealing the shaft and sleeve to remove these stresses
also anneals away desirable mechanical properties in the shaft and
changes the magnetic properties of the sleeve. Furthermore, like
the Dahle device, the shortcomings of Scoppe's transducer, due to
its need to monitor permeability changes lying along the principal
stress directions, are its sensitivity to variations in its radial
distance from the shaft, magnetic inhomogeneity around the shaft
circumference and dependence on shaft speed.
A different approach to magnetoelastic torque sensing utilizes the
differential magnetic response of two sets of amorphous
magnetoelastic elements adhesively attached to the torqued shaft.
This approach has the advantage over prior approaches that it is
insensitive to rotational position and shaft speed. However, it
requires inordinate care in the preparation and attachment of the
elements. Moreover, transducer performance is adversely affected by
the methods used to conform the ribbon elements to the shape of the
torqued member; the properties of the adhesive, e.g., shrinkage
during cure, expansion coefficient, creep with time and temperature
under sustained load; and, the functional properties of the
amorphous material itself, e.g., consistency, stability. Still
another concern is in the compatibility of the adhesive with the
environment in which the transducer is to function, e.g., the
effect of oil, water, or other solvents or lubricants on the
properties of the adhesive.
In the article "A New Torque Transducer Using Stress Sensitive
Amorphous Ribbons", IEEE Trans. on Mag., MAG-18, No. 6, 1767-9,
1982, Harada et al. disclose a torque transducer formed by gluing
two circumferential stress-sensitive amorphous ribbons to a shaft
at axially spaced apart locations. Unindirectional magnetoelastic
magnetic anisotropy is created in each ribbon by torquing the shaft
in a first direction before gluing a first ribbon to it, releasing
the torque to set-up stresses within the first ribbon, torquing the
shaft in the opposite direction, gluing the second ribbon to it,
and then releasing the torque to set-up stresses within the second
ribbon. The result is that the anisotropy in one ribbon lies along
a right-hand helix at +45.degree. to the shaft axis while the
anisotropy in the other ribbon lies along an axially symmetric
left-hand helix at -45.degree. to the shaft axis. AC powered
excitation coils and sensing coils surround the shaft making the
transducer circularly symmetric and inherently free from
fluctuation in output signal due to rotation of the shaft. In the
absence of torque, the magnetization within the two ribbons will
respond symmetrically to equal axial magnetizing forces and the
sensing coils will detect no difference in the response of the
ribbons. However, when torque is applied, the resulting stress
anisotropy along the principal axes arising from the torque
combines asymmetrically with the quiescent anisotropies previously
created in the ribbons and there is then a differing response of
the two ribbons to equal axial magnetizing force. This differential
response is a function of the torque and the sensing coils and
associated circuitry provide an output signal which is proportional
to the applied torque. Utilizing substantially the same approach,
in Japanese patent publication No. 58-9034, two amorphous ribbons
are glued to a shaft and symmetrical magnetic anisotropy is given
to the ribbons by heat treatment in a magnetic field at
predetermined equal and opposite angles. Amorphous ribbons have
also been glued to a shaft in a .+-.45.degree. chevron pattern, see
Sasada et al., IEEE Trans. on Mag., MAG-20, No. 5, 951-53, 1984,
and amorphous ribbons containing parallel slits aligned with the
.+-.45.degree. directions have been glued to a shaft, see, Mohri,
IEEE Trans. on Mag., MAG-20, No. 5, 942-47, 1984, to create shape
magnetic anisotropy in the ribbons rather than magnetic anisotropy
due to residual stresses. Other recent developments relevant to the
use of adhesively attached amorphous ribbons in a magnetoelastic
torque transducer are disclosed in U.S. Pat. No. 4,414,855--Iwasaki
and U.S. Pat. No. 4,598,595--Vranish et al.
More recently, in apparent recognition of the severe shortcomings
inherent in using adhesively affixed ribbons, plasma spraying and
electrodeposition of metals over appropriate masking have been
utilized. See: Yamasaki et al, "Torque Sensors Using Wire Explosion
Magnetostrictive Alloy Layers", IEEE Trans. on Mag., MAG-22, No. 5,
403-405 (1986); Sasada et al, "Noncontact Torque Sensors Using
Magnetic Heads and Magnetostrictive Layer on the Shaft
Surface--Application of Plasma Jet Spraying Process", IEEE Trans.
on Mag., MAG-22, No. 406-408 (1986).
The hereinbefore described work with amorphous ribbons was not the
first appreciation that axially spaced-apart circumferential bands
endowed with symmetrical, helically directed magnetic anisotropy
contributed to an improved torque transducer. USSR Certificate No.
667,836 discloses a magnetoelastic torque transducer having two
axially spaced-apart circumferential bands on a shaft, the bands
being defined by a plurality of slots formed in the shaft in a
.+-.45.degree. chevron pattern, and a pair of excitation and
measuring coil-mounting circumferential bobbins axially located
along the shaft so that a band underlies each bobbin. The shape
anisotropy created by the slots is the same type of magnetic
preconditioning of the shaft as was created, for example, by the
chevron-patterned amorphous ribbons of Sasada et al and the slitted
amorphous ribbons of Mohri, and suffers from many of the same
shortcomings. USSR Certificate No. 838,448 also discloses a
magnetoelastic torque transducer having two spaced-apart
circumferential bands on a shaft, circumferential excitation coils
and circumferential measuring coils surrounding and overlying the
bands. In this transducer the bands are formed by creating a knurl
in the shaft surface with the troughs of the knurl at
.+-.45.degree. angles to the shaft axis so that the troughs in one
band are orthogonal to the troughs in the other band. The knurls
are carefully formed by a method which ensures the presence of
substantial unstressed surface sections between adjacent troughs so
that the magnetic permeability of the troughs is different from the
magnetic permeability of the unstressed areas therebetween.
Inasmuch as the trough width-to-pitch ratio corresponds to the
stressed to unstressed area ratio and the desired ratio appears to
be 0.3, there is no circumferential region in either band which is
intentionally stressed over more than 30% of its circumferential
length. This very minimal stress anisotropic preconditioning is
believed to be too small to provide a consistent transducer
sensitivity, as measured by the electronic signal output of the
measuring coils and their associated circuitry, for economical
commercial utilization.
Notwithstanding their many shortcomings in forming sensitive and
practical bands of magnetic anisotropy on a torqued shaft, the
efforts evidenced in the Harada et al, Sasada et al, Mohri and
Yamasaki et al articles and the USSR certificates represent
significant advances over the earlier work of Beth and Meeks, Dahle
and Scoppe in recognizing that a pair of axially spaced-apart,
circumferential bands of symmetrical, helically directed anisotropy
permits averaging axial permeability differences over the entire
circumferential surface. This is notably simpler than attempting to
average helical permeability differences sensed along the principal
stress axes, as had earlier been suggested. Moreover, neither
rotational velocity nor radial eccentricity significantly influence
the permeability sensed in this manner. Nevertheless, these efforts
to perfect means of attachment of magnetoelastically optimized
material to the surface of the torqued member introduces
unacceptable limitations in the resulting torque sensor. The
application to the shaft of adhesively affixed amorphous ribbons
suffers from significant drawbacks, such as the methods used to
conform the ribbons to the shaft, the properties of the adhesive
and the functional properties of the amorphous material, which make
such ribbons impractical for commercial implementation. The use of
rigidly affixed sleeves as taught by Scoppe and, more recently, in
U.S. Pat. No. 4,506,554--Blomkvist et al, is unsuitable for
practical applications due to the higher costs involved as well as
the stresses created by high temperature welding and/or the
uncertainties in magnetic and mechanical properties created by
subsequent annealing. Likewise, reliance upon shape anisotropy or
predominantly unstressed regions to create stress anisotropy
present significant problems which make such techniques impractical
for commercial implementation.
It is, therefore, apparent that despite the many advances in torque
transducer technology, there still exists a need for a
magnetoelastic torque transducer which is significantly more
economical than previous torque transducers, allowing use in many
applications for which such transducers were not heretofore either
economically or environmentally viable, and which is applicable to
small as well as large diameter shafts, whether stationary or
rotating at any practical speed.
DISCLOSURE OF THE INVENTION
In accordance with one broad aspect of the present invention there
is provided a magnetoelastic torque transducer for providing an
electrical signal indicative of the torque applied to a member in
which a ferromagnetic and magnetostrictive region of the torqued
member serves as a part of the magnetic sensing circuit of the
transducer by providing at the surface of said region a pair of
axially spaced-apart annular bands endowed with residual stress
created, respectively symmetrical, left and right hand helically
directed magnetic anisotropy of relatively large magnitude, which
anisotropy overwhelms and/or renders negligible or insignificant
any random anisotropy in the member as a result of internal
stresses due to mechanical working, inhomogeneities, crystal
orientation, and the like.
In accordance with another aspect of the present invention, there
is provided a magnetoelastic torque transducer for providing an
electrical signal indicative of the torque applied to a member,
said member having a ferromagnetic and magnetostrictive region,
said transducer comprising a pair of axially spaced-apart annular
bands defined within said region, said bands having, at least at
the surface of said member, respectively symmetrical right and left
hand helically directed residual stress created magnetic
anisotropy, each said band having at least one circumferential
region which is free of residually unstressed areas, i.e., said at
least one circumferential region is residually stressed, over at
least 50% of its circumferential length; means for applying an
alternating magnetic field to said bands; means for sensing the
change in permeability of said bands caused by said applied torque;
and means for converting said sensed change in permeability to an
electrical signal indicative of the magnitude of the torque applied
to said member. In a preferred aspect, the ferromagnetic and
magnetostrictive region is formed of nickel maraging steel.
In accordance with another aspect, the present invention
contemplates a magnetoelastic torque transducer for providing an
electrical signal indicative of the torque applied to a member,
including ferromagnetic, magnetostrictive means rigidly affixed to,
associated with or forming a part of the surface of said torqued
member for altering in magnetic permeability in response to the
application of torque to said member, means for applying a magnetic
field to said ferromagnetic, magnetostrictive means, means for
sensing the change in permeability caused by said applied torque
and means for converting said sensed change in permeability to an
electrical signal indicative of the magnitude of the torque applied
to said member, the ferromagnetic, magnetostrictive means being
formed from nickel maraging steel.
In still another aspect of the present invention, there is provided
a method of sensing the torque applied to a member having a
ferromagnetic and magnetostrictive region, which includes the steps
of endowing a pair of axially spaced-apart annular bands within
said region with respectively symmetrical, right and left hand
helically directed magnetic anisotropy, applying an alternating
magnetic field to said bands and sensing the permeability
difference between said bands resulting from the application of
torque to said member, the difference being indicative of the
magnitude of the applied torque, the improvement which comprises
forming said bands at the surface of said member and endowing said
bands with magnetic anisotropy by instilling a residual stress
distribution in each band which is sufficiently extensive that at
least one circumferential region within each band is free of
residually unstressed areas, i.e., said at least one
circumferential region is residually stressed, over at least 50% of
its circumferential length. In a preferred aspect of this method,
the ferromagnetic and magnetostrictive region is formed from nickel
maraging steel.
In yet another aspect of the invention, there is provided a method
of sensing the torque applied to a member having a ferromagnetic
and magnetostrictive region which includes the steps of endowing
said region with helically directed magnetic anisotropy by
instilling a residual stress distribution in said region which is
sufficiently extensive that at least one circumferential region
within said ferromagnetic and magnetostrictive region is free of
residually unstressed areas, i.e., said at least one
circumferential region is residually stressed, over at least 50% of
its circumferential length, applying an alternating magnetic field
to said ferromagnetic and magnetostrictive region and to an area of
said member not so endowed, and sensing the permeability difference
between said ferromagnetic and magnetostrictive region and said
area resulting from the application of torque to said member, the
difference being indicative of the magnitude of the applied torque.
In a preferred aspect of this method, the ferromagnetic and
magnetostrictive region is formed from nickel maraging steel.
BRIEF DESCRIPTION OF THE DRAWINGS
The invention will be better understood from the following
description taken in conjunction with the accompanying drawings in
which:
FIG. 1 is a perspective view of a magnetoelastic torque transducer
in accordance with the present invention;
FIG. 2 is a sectional view of a magnetoelastic torque transducer in
accordance with the present invention illustrating one form of
magnetic discriminator useful therewith;
FIG. 3 is a circuit diagram showing the circuitry associated with
the magnetic discriminator of FIG. 2;
FIG. 4 is a schematic view of a magnetoelastic torque transducer in
accordance with the present invention illustrating another form of
magnetic discriminator, and its associated circuitry, useful
therewith;
FIG. 5 is a graphical representation of the relationship between
applied torque and output signal for several magnetoelastic torque
transducers of the present invention;
FIG. 6 is a graphical representation of the relationship between
applied torque and output signal for the magnetoelastic torque
transducers of FIG. 5 after the shafts thereof have been heat
treated under identical conditions;
FIG. 7 is a graphical representation of the general relationship
between torque transducer sensitivity and residual stress loading
along the circumferential length of a circumferential region of the
bands of a transducer of the present invention;
FIG. 8 is an elevational view of a test piece used in torque
transducer sensitivity testing; and
FIG. 9 is a graphical illustration, as in FIG. 7, of the
sensitivity vs. residual stress loading relationship for a
transducer of the present invention wherein the bands thereof were
endowed with residual stress induced magnetic anisotropy by a
controlled knurling technique.
BEST MODE FOR CARRYING OUT THE INVENTION
In accordance with the present invention there is provided a
magnetoelastic torque transducer comprising (1) a torque carrying
member at least the surface of which, in at least one complete
circumferential region of suitable axial extent, is appropriately
ferromagnetic and magnetostrictive; (2) two axially distinct
circumferential bands within this region or one such band in each
of two such regions that are endowed with respectively symmetrical,
helically directed residual stress induced magnetic anisotropy such
that, in the absence of torque, the magnetization tends to be
oriented along a left-hand (LH) helix in one band and along an
axially symmetrical right-hand (RH) helix in the other band; and
(3) a magnetic discriminator device for detecting, without
contacting the torqued member, differences in the response of the
two bands to equal, axial magnetizing forces.
These features of the magnetoelastic torque transducer of the
present invention will be better understood by reference to FIG. 1
in which a cylindrical shaft 2 formed of ferromagnetic and
magnetostrictive material or, at least having a ferromagnetic and
magnetostrictive region 4, is illustrated having a pair of axially
spaced-apart circumferential or annular bands 6,8 endowed with
respectively symmetrical, helically directed magnetic stress
anisotropy in the angular directions .+-..theta. of the respective
magnetic easy axes 10,12. A magnetic discriminator 14 is spaced
from shaft 2 by a small radial space. In the absence of applied
torque the magnetization within the bands 6,8 will respond
symmetrically to the application of equal axial magnetizing forces.
Longitudinal or axial components of the magnetization within these
two bands remain identical, since cos .theta.=cos-(.theta.) for all
values of .theta., and the magnetic discriminator will therefore,
detect no difference or zero. With the application of torque to
shaft 2, the stress anisotropy arising therefrom combines
asymmetrically with the quiescent anisotropies intentionally
instilled in the bands and there is then a differing response of
the two bands to equal axial magnetizing force. Since the stress
anisotropy is a function of the direction and magnitude of the
torque, the differential response of the two bands will be a
monotonic function of the torque. The resulting differences in
magnetic anisotropy in each of the bands is evidenced by the axial
permeability of one band increasing and that of the other band
decreasing. The difference in axial permeabilities of the two bands
is used to sense the torque. A properly designed magnetic
discriminator will detect detailed features of the differential
response and provide an output signal that is an analog of the
torque.
In accordance with the present invention, the torque carrying
member is provided with two axially spaced-apart, distinct
circumferential or annular bands in the ferromagnetic region of the
member. There are no particular geometric, space, location or
circumferential limitations on these bands, save only that they
should be located on the same diameter member and close enough to
one another to experience the same torque. The bands are
intentionally endowed with respective symmetrical, helically
directed, magnetic anisotropy caused by residual stress. Residual
stress may be induced in a member in many different ways, as
discussed more fully hereinbelow. However, all techniques have in
common that they apply stress to the member beyond the elastic
limit of at least its surface region such that, when the applied
stress is released, the member is unable to elastically return to
an unstressed condition. Rather, residual stresses remain which, as
is well known, give rise to magnetic anisotropy. Depending upon the
technique utilized for applying stress, the angular direction of
the tangential principal residual stress with the member's axis
will vary between values greater than zero and less than
90.degree.. Preferably, the angular direction of the residual
stress and that of the resulting magnetic easy axes, is from
10.degree.-80.degree. and, most desirably, from
20.degree.-60.degree..
It will be appreciated that inasmuch as the sensing of torque is
primarily accomplished by sensing the change in permeability at the
surface of the torqued member, it is at least at the surface of
each band that there must be magnetic anisotropy created by
residual stress. Hence, the limitation that the applied stress must
be at least sufficient to exceed the elastic limit, of the member
at its surface. It will, of course, be appreciated that the
application of an applied stress exceeding the minimum will,
depending upon the magnitude of the applied stress, result in
residual stress within the body of the member as well. For use
herein, the term "surface" of the member means at the surface and
within 0.010 inch thereof.
Any method of applying stress to a member to exceed the elastic
limit thereof at the surface of the bands may be employed which
produces uneven plastic deformation over the relevant cross-section
of the member. Thus, the residual stress including method may be
mechanical, thermal, or any other which is suitable. It is
particularly desirable that the residual stress-inducing applied
stress exceed the maximum expected applied stress when the member
is torqued in use. This is to insure that torquing during use does
not alter the residual stress pattern and, thus, the magnetic
anisotropy within the bands. The residual stress induced in the
respective bands should be substantially equal and symmetrical in
order that axial permeability sensing, when equal axial magnetizing
forces are applied to the member, will produce a "no difference"
output in the untorqued condition and equal but opposite output as
a result of the application of equal clockwise (CW) and
counter-clockwise (CCW) torques.
The method chosen to apply stress to a member beyond the elastic
limit thereof in order to create residual stress is largely a
function of the member's size, shape, material and intended
application. The method may induce continuous and substantially
equal residual stresses over the entire surface of the band, i.e.,
around the entire band circumference and along its entire axial
length. Alternatively, the method may induce a residual stress
pattern within each band which includes both stressed and
unstressed areas. Such a pattern, however, is subject to the
important limitation that each band must have at least one
continuous circumferential region which is free of unstressed areas
over at least 50% of its circumferential length, desirably over at
least 80% of its circumferential length. In a particularly
preferred configuration, each band would have at least one
continuous circumferential region which is free of unstressed areas
over its entire circumferential length. As a general matter, it is
particularly desirable to maximize the amount of shaft surface
which is intentionally stressed in order to endow as much of the
surface as is possible with relatively large magnitude controlled
magnetic anisotropy. This leaves as little of the shaft surface as
possible subject only to the random anisotropies created during
shaft manufacture, due to internal stresses and resulting from
crystal orientation. It should be appreciated that the problems
associated with random anisotropy inherent in using the shaft
itself as an operative element, i.e., the sensing region, of the
magnetic circuit of the torque sensor are overcome, in accordance
with the present invention, by replacing and/or overwhelming the
random anisotropy with relatively large magnitude intentionally
created residual stress induced anisotropy. For obvious reasons,
the greater the intentionally induced anisotropy, the less
significant is any residual random anisotropy.
As used hereinbefore and hereinafter, the term "circumferential
region" means the locus of points defining the intersection of (1)
a plane passing perpendicular to the member's axis and (2) the
surface of the member, as hereinbefore defined. Where the member is
a cylindrical shaft, the circumferential region is a circle
defining the intersection of the cylindrical surface with a plane
perpendicular to the shaft axis, and such a circle has a
circumference or circumferential length. Stated otherwise, if each
element of the member's surface comprising the circumferential
region were examined, it would be seen that each such element was
either stressed or unstressed. In order to form a commercially
functional torque sensor having broad applicability, particularly
in small diameter shaft applications, which exhibits acceptable and
commercially reproducible sensitivity, linearity and output signal
strength, it has been found that at least 50% of these elements
must have been stressed beyond their elastic limit and, therefore,
must remain residually stressed after the applied stress is
removed.
The range of methods by which torque carrying members can be
endowed with the desired bands containing residual stress instilled
helically directed magnetic easy axes, i.e., directions in which is
easiest, is virtually endless. From the point of view of transducer
performance the most important consideration is the adequacy of the
resulting anisotropy, i.e., the band anisotropy created must be at
least of comparable magnitude to the stress anisotropy contributed
by the applied torque. From the point of view of compatibility with
the device in which the transducer is installed, the compelling
consideration is consequential effects on the member's prime
function. Other important considerations in selecting a method are
practicality and economics. Examples of suitable methods for
imprinting residual stress induced magnetically directional
characteristics on, i.e., at the surface of, a torque carrying
member include, but are not limited to torsional overstrain;
knurling, grinding, mechanical scribing; directed or masked shot
peening or sand blasting; roll crushing; appropriate chemical
means; selective heat treatments, e.g., induction, torch, thermal
print head, laser scribing.
Of the foregoing, the creation of areas of residual stress by
torsional overstrain has been found to be a simple, economical and
effective method for small diameter shafts. It is particularly
desirable because it neither distorts nor interrupts the surface of
the shaft and is, therefore, compatible with virtually any
application. However, the manner of applying torsional overstrain,
e.g., by twisting both sides of a centrally restrained region,
makes it impractical for and inapplicable to large diameter shafts
formed of high elastic limit materials. Knurling is a desirable
manner of inducing residual stress in a shaft of virtually any
diameter. With knurling, the exact location of the bands, their
axial extent, separation and location can be closely controlled. In
addition, knurling allows relatively simple control of the helix
angles of the easy axes. Very importantly, knurling permits
predetermination of the salient features of the knurl itself, such
as pitch, depth and cross-sectional shape and, thereby, allows
control of the residual stress induced. It should be appreciated
that, in accordance with the present invention, enough of the
surface of each band must be stressed that there exists within each
band at least one continuous cicumferential region which is free of
unstressed areas over at least 50% of its circumferential length.
Not all knurling is this extensive and care must be taken to select
a knurl which achieves this objective. Inasmuch as knurling
disrupts the surface of the shaft in order to form the knurl
thereon, a knurled band is endowed with shape anisotropy as well as
residual stress anisotropy. If it is desired, for example, for
compatibility of the knurled shaft with an intended application,
the gross shape features of the knurl may be ground off the shaft
to leave only magnetic anisotropy caused by residual stress. Other
forms of cold working, with or without surface deformation,
likewise create residual stress and associated magnetic anisotropy.
In addition, more sophisticated methods, such as electron beam and
laser scribing as well as selective heat treatment can provide the
desired anisotropy with less mutilation of the shaft surface than
most mechanical cold working methods. Moreover, these methods offer
the opportunity of very close control of the induced residual
stresses by adjustment of the power density and intensity of the
beam and/or the thermal gradients.
Whatever method may be selected for creating residual stress within
the bands, it should be appreciated that the relationship between
the percent of stressed areas along the circumferential length of a
circumferential region within each band ("% stressed areas") and
sensitivity (in millivolts/N-M) is one wherein the sensitivity
increases with increasing "% stressed areas". A plot of these
parameters yields a curve which has its greatest slope at the lower
values of "% stressed areas" and which has a decreasing slope at
the higher values of "% stressed areas", up to 100%, at which point
the sensitivity is greatest and the slope is close to zero. The
precise shape of the curve, its slope for any particular value of
"% stressed areas", its initial rate of ascent and the point at
which the rate of ascent decreases and the curve levels off are all
functions of the material of the bands and the manner in which the
stress is applied. A typical curve is shown in FIG. 7. At "A",
there is no residual stress along the circumferential length of the
circumferential region. At "C", 100% of the circumferential length
of the circumferential region is subjected to residual stress. "B"
represents the approximate point on the curve at which sensitivity
begins to level off, i.e., becomes less responsive to "% stressed
areas," a point which is both material and method dependent.
Ideally, torque sensor operation at 100% residual stress, i.e., at
"C" on the curve, is best because the rate of change of sensitivity
is minimized and the 100% stressed condition is generally easiest
to attain with most methods. As a practical matter, it is difficult
to control the residual stress inducing method to achieve a value
for desired "% stressed area" which is less than 100%. However,
practical production problems aside, acceptable torque sensors can
be made which operate at sensitivity levels corresponding to less
than 100% residual stress along the length of a circumferential
region of the bands.
Torque sensors cannot economically and reproducibly be made to
operate in the ascending portion AB along the curve in FIG. 7
since, in that portion, the sensitivity is extremely responsive to
"% stressed areas". This means that even small changes in "%
stressed areas" causes relatively large changes in sensitivity.
From a practical, commercial standpoint, mass produced torque
sensors must have a known and reproducible sensitivity. It would be
unrealistic to have to individually calibrate each one. However,
even normal production inconsistencies will cause small "% stressed
areas" changes which will result, in the AB region of the curve, in
large sensitivity differences among sensors. Therefore,
commercially useful torque sensors have to operate along a flatter
portion of the curve, where the slope is closer to zero. Operating
in the BC portion of the curve appears to be an acceptable
compromise. It is preferred, for most materials and residual stress
inducing methods, that the point represented by "B" exceed at least
50%, preferably at least 80%, stressed areas along the
circumferential length of a circumferential region. This is in
recognition of the fact that the minimum acceptable residual stress
loading of a circumferential region is both material and process
dependent and that it is generally most desirable to be as close to
100% stress loading as is practical.
To demonstrate the applicability of the foregoing in fabricating an
operable torque sensor, with reference to FIG. 8, a 0.25 inch OD
cylindrical shaft 100 was formed with two shoulders 102 of equal
axial length spaced apart by a reduced diameter shaft portion 104
of 0.215 inch OD. The shaft was formed of a nickel maraging steel
commercially available as Unimar 300K from Universal-Cyclops
Specialty Steel Division, Cyclops Corporation of Pittsburgh, Pa.
and was pre-annealed at 813.degree. C. in hydrogen to relieve
internal stresses. Each shoulder 102 was carefully knurled using a
pair of identical 3/4 inch OD, 3/8 inch long knurling rollers
having 48 teeth around their circumference. The shoulders were
brought into contact with the knurling rollers in a controlled
manner to form symmetrical knurls on each shoulder at angles of
.+-.30.degree. to the shaft axis. Careful control of the infeed of
the tool relative to the shouldes allowed the axial width and depth
of each knurl trough to be controlled. The "% stressed areas" along
the circumferential length of a circumferential region of each
knurled shoulder was determined by assuming that the knurl trough
was the only stressed area on the shoulder and that the shoulder
surface between troughs was unstressed by the knurling operation;
by measuring the trough width and chordal knurl pitch and
converting the chordal pitch to circumferential pitch; and by
calculating the trough width to circumferential pitch ratio, which
ratio when multiplied by 100 represented the desired "% stressed
areas" value. The shaft prepared in this manner was affixed to a
lever are which permitted 10-one pound weights to be suspended from
cables at each end of the arm. The lever arm was so dimensioned
that addition or removal of a single one pound weight from either
side represented a torque change on the shaft of 0.5N-M. By
appropriate shifting of the weights, the torque on the shaft could
be altered in both magnitude and direction.
FIG. 9 graphically illustrates the relationship between "% stressed
areas" and sensitivity for a shaft prepared as described
hereinabove. It can be seen that the curve ascends rapidly up to
about 60% stress loading and then appears to level off rather
rapidly thereafter. This is because there is believed to be a
greater correlation at lower "% stressed area" values between the
trough width to circumferential pitch ratio and the actual
percentage of stressed areas along the circumferential length of a
circumferential region of the shaft. As the width and depth of the
knurling trough increases it becomes apparent that the shoulder
surface between troughs, at least in the vicinity of the trough
edges, becomes slightly deformed and, more than likely, residually
stressed. Therefore, the point on the curve at which 100% stress
loading in a circumferential region is actually achieved is
somewhat less than the calculated 100% value, accounting for the
rapid flattening of the curve at the higher "% stressed areas"
portions therof. This suggests that, with many processes, such as
knurling, the 100% stress loading point can be achieved with less
than 100% topographic disruption. It will be appreciated in this
connection, that each method of inducing residual stress in a shaft
will produce its own distinctive curve of "% stressed areas" vs.
sensitivity, although it is believed that each curve will have the
same general characteristics as appear in FIGS. 7 and 9.
In accordance with the foregoing, it can be seen that in the
absence of applied torque, the application to the bands of equal
axial magnetizing forces causes the bands to respond symmetrically
and the sensing means associated with the bands detect no
difference in response. When torque is applied, the principal
stresses associated with the applied torque combine with the
residual stresses in the bands in such a manner that the resultant
stresses in the two bands are different from each other. As a
result, the magnetic permeabilities are different and the emf
induced in the sensing means associated with each band reflect that
difference. The magnitude of the difference is proportional to the
magnitude of the applied torque. Thus, the instant system senses a
differential magnetoelastic response to the principal stresses
associated with the applied torque between two circumferential
bands. The significance of this is that sensing in this manner
amounts to sensing the response average over the entire
circumference of the band. In this manner, sensitivity to surface
inhomogenity, position and rotational velocity are avoided.
This sensing of magnetic permeability changes due to applied torque
can be accomplished in many ways, as is disclosed in the prior art.
See, for example, the aforementioned article of Harada et al and
U.S. Pat. No. 4,506,554. Functionally, the magnetic discriminator
is merely a probe for assessing any differential magnetoelastic
response to applied torque between the two bands. In general, it
functions by imposing equal magnetizing forces on both bands and
sensing any differences in their resulting magnetization. The
magnetizing forces may come from electrical currents, permanent
magnets, or both. Resulting magnetization may be sensed through its
divergence, either by the resulting flux or its time rate of
change. The transducer function is completed by the electrical
circuitry which delivers an electrical signal that is an analog of
the torque.
One method of supplying the magnetization forces and for measuring
the resulting difference signal from the sensing coil is shown in
FIGS. 2 and 3. Referring to FIG. 2, it can be seen that the bands
6,8 are surrounded by bobbins 16,18 which are concentric with shaft
2. Mounted on bobbins 16,18 are a pair of coils 20,22 and 24,26 of
which 22 and 26 are excitation or magnetizing coils connected in
series and driven by alternating current and 20 and 24 are
oppositely connected sensing coils for sensing the difference
between the fluxes of the two bands. A ferrite material core 28 is
optionally provided as a generally E-shaped solid of revolution.
Circumferential gaps 30 between the shaft and the E-shape core are
desirably maintained as small and uniform as is practical to
maintain the shaft centered within the core. FIG. 3 shows that
excitation or drive coils 22,26 are supplied in series from AC
source 32 and the emf induced in the oppositely connected sensing
coils 20,24 is phase sensitively rectified in the rectifier 34 and
is displayed on voltage display instrument 36. Black dots 38
indicate the polarity of the coils.
Inasmuch as the stresses in the bands are symmetrical and equal
when no torque is applied to shaft 2, under these conditions the
output signal from the circuitry shown in FIG. 3 will be zero,
regardless of the applied a.c. driving input. This is because the
bands have equal magnetic permeability. Thus the voltages induced
in the sensing coils are equal in magnitude and opposite in
polarity and cancel each other. However, when a torque is applied
to shaft 2, the respective bands will be subjected to tensile and
compressive stresses, with a resulting increase of permeability and
of the flux passing through one of the bands, and a resulting
decrease of permeability and of the flux passing through the other
of the bands. Thus, the voltage induced in one of the sensing coils
will exceed the voltage induced in the other sensing coil and an
output signal representing the difference between the induced
voltages and proportional to the applied torque will be obtained.
The signal is converted to a direct current voltage in the
rectifier 34 and the polarity of the rectifier output will depend
upon the direction, i.e., CW or CCW, of the applied torque.
Generally, it has been found that in order to obtain linear, strong
output signals, the a.c. driving current should advantageously be
maintained in the range 10 to 400 milliamperes at excitation
frequencies of 1 to 100 kHz.
FIG. 4 illustrates another type of magnetic discriminator for
sensing the permeability change of the bands upon application of a
torque to the shaft. Magnetic heads 42,44 comprising a
ferromagnetic core and a coil wound thereupon are provided in axial
locations along shaft 40 which coincide with bands 46,48 and are
magnetically coupled to the bands. The magnetic heads 42,44 are
excited by high frequency power source 50 through diodes 52,54.
With no torque applied to shaft 40, the magnetic permeability of
the bands are equal. Therefore, the inductance levels of both
magnetic heads are equal and opposite in polarity, and the net
direct current output, V.sub.out, is zero. When torque is applied
to shaft 40, as shown by arrows 60, the magnetic permeability of
one band increases while the permeability of the other decreases.
Correspondingly, the inductance of one magnetic head increases
while the inductance of the other decreases, with a resultant
difference in excitation current between the heads. This difference
in excitation current, passed via output resistors 56 and smoothing
capacitor 58, produces a direct current output signal which has
polarity and magnitude indicative of the magnitude and direction of
the applied torque.
In accordance with one unique aspect of the present invention, as
hereinbefore described, a shaft of suitable material is endowed in
each of two proximate bands with symmetrical, left and right handed
helical magnetic easy axes. At least in the region of the bands,
and more commonly over its entire length the shaft is formed, at
least at its surface, of a material which is ferromagnetic and
magnetostrictive. The material must be ferromagnetic to assure the
existence of magnetic domains and must be magnetostrictive in order
that the orientation of the magnetization may be altered by the
stresses associated with an applied torque. Many materials are both
ferromagnetic and magnetostrictive. However, only those are
desirable which also exhibit other desirable magnetic properties
such as high permeability, low coercive force and low inherent
magnetic anisotropy. In addition, desirable materials have high
resistivity in order to minimize the presence of induced eddy
currents as a result of the application of high frequency magnetic
fields. Most importantly, favored materials must retain these
favorable magnetic properties following the cold working and heat
treating necessary to form them into suitable shafts having
appropriately high strength and hardness for their intended
use.
It is true that many high strength steel alloys are ferromagnetic
and magnetostrictive. However, to varying degrees, the vast
majority of these alloys experience a degradation in their magnetic
properties as a result of the heat treating necessary to achieve
suitable hardness and strength for the desired application. The
most significant degradation is noted in those alloys hardened by
carbon or carbides for which the conventional inverse relationship
between mechanical hardness and magnetic softness appears to have a
sound basis. However, the performance of even low carbon alloys
such as AISI 1018 is found to significantly degrade with heat
treating. The same is true for martensitic stainless steels, e.g.,
AISI 410, and highly alloyed steels, e.g., a 49Fe-49Co-2V alloy. It
has been determined, in accordance with another unique aspect of
the present invention, that the nickel maraging steels possess the
unusual combination of superior mechanical properties and
outstanding and thermally stable magnetic properties which give
them a special suitability and make them particularly advantageous
for use in all magnetoelastic torque transducers in which a
magnetic field is applied to ferromagnetic, magnetostrictive means
and the change in permeability caused by torque applied thereto is
sensed to obtain an indication of the magnitude of the applied
torque. This is the case whether the ferromagnetic,
magnetostrictive means is affixed to, associated with or forms a
part of the surface of the torqued member and whether or not the
ferromagnetic, magnetostrictive means is endowed with bands of
intentionally instilled magnetic anisotropy and irrespective of the
number of bands which may be used.
The nickel maraging steels are, typically, extra-low-carbon, high
nickel, iron-base alloys demonstrating an extraordinary combination
of structural strength and fracture toughness in a material which
is readily weldable and easy to heat-treat. They belong to a
loosely knit family of iron-base alloys that attain their
extraordinary strength characteristics upon annealing, by
transforming to an iron-nickel martensitic microstructure, and
following cooling, upon aging in the annealed or martensitic
condition. Thus, the alloys are termed "maraging" because of the
two major reactions involved in their strengthening--martensitizing
and aging. However, these steels are unique due to their high
nickel and extremely low carbon content, which permits formation of
an outstandingly tough martensite that can be strengthened rapidly
to extraordinarily high levels. Yield strengths up to and well
beyond 300 ksi are available in these steels in the aged
condition.
Typical nickel maraging steels are alloys comprising 12-25% Ni,
7-13% Co, 2.75-5.2% Mo, 0.15-2.0% Ti, 0.05-0.3% Al, up to 0.03% C,
balance Fe and incidential amounts of other elements, such as Mn,
Si, S, P, Cb. The most popular and practically significant maraging
steels, at least at present, are the 18% Ni steels which can be
aged to develop yield strengths of about 200 ksi, 250 ksi and 300
ksi. These particular alloys, referred to as 18Ni200, 18Ni250 and
18Ni300 grade maraging steels have typical compositions in the
ranges 17-19% Ni, 7-9.5% Co, 3.0-5.2% Mo, 0.1-0.8% Ti, 0.05-0.15%
Al, up to 0.03% C, balance Fe and incidential amounts of other
elements. Typically, the 18% nickel maraging steels are heat
treated by annealing at temperatures of 1500.degree. F. and above
for a sufficient time, e.g., one hour per inch of thickness, to
dissolve precipitates, relieve internal stresses and assure
complete transformation to austenite. Following air cooling, the
18% Ni steels are conventionally aged at 750.degree.-1100.degree.
F., desirably 900.degree.-950.degree. F., for 3 to 10 hours,
depending upon thickness, usually 3-6 hours. However, it has been
found that satisfactory strength characteristics and superior
magnetic characteristics can be attained in alloys aged for as
little as 10 minutes.
Other well known nickel maraging steels are cobalt-free 18% Ni
maraging steels as well as cobalt-containing 25% Ni, 20% Ni and 12%
Ni maraging steels. The 18% Ni-cobalt containing maraging steels
are commercially available from a number of sources. Thus, such
steels are obtainable under the trademarks VascoMax C-200, VascoMax
C-250, VascoMax C-300 and VascoMax C-350 from Teledyne Vasco of
Latrobe, Pa.; under the trademarks Marvac 250 and Marvac 300 from
Latrobe Steel Company of Latrobe, Pa.; under the trademark Unimar
300K from Universal-Cyclops Specialty Steel Division, Cyclops
Corporation of Pittsburgh, Pa.; and, under the trademark Almar
18-300 from Superior Tube of Norristown, Pa. The 18% Ni-cobalt free
maraging steels are commercially available under the trademarks
VascoMax T-200, VascoMax T-250 and VascoMax T-300 from Teledyne
Vasco of Latrobe, Pa. Other high nickel steels which form an
iron-nickel martensite phase exhibit mechanical and magnetic
properties which are similar to those of the more conventional
maraging steels and which are also substantially stable to
temperature variations. Most notable among these is a nominally 9%
Ni-4% Co alloy available from Teledyne Vasco having a typical
composition, in percent by weight, of 9.84 Ni, 3.62 Co, 0.15 C,
balance Fe. In addition, maraging steels of various other high
nickel-cobalt compositions, e.g., 15% Ni-15% Co, are continuously
being tested in efforts to optimize one or another or some
combination of properties. Therefore, as used herein, the term "Ni
maraging steel" refers to alloys of iron and nickel which contain
from 9-25% nickel and which derive their strength characteristics
from iron-nickel martensite formation, as hereinbefore
described.
In addition to their outstanding physical and strength
characteristics, the nickel maraging steels have excellent magnetic
properties which make them outstanding for use as the magnetic
material in non-contact torque transducers. Thus, they have high
and substantially isotropic magnetostriction, in the range of 25
ppm.+-.15 ppm, and do not exhibit a Villari reversal; high
electrical dresistivity; low inherent magnetic anisotropies due to
crystalline structure; high magnetic permeability; low coercive
force, in the range 5-25 oersted; and, stability of magnetic
properties with alloy chemistry. However, most important is that
their magnetic properties are only modestly, yet favorably,
affected by strengthening treatments--indeed, their magnetic
properties improve with cold work and aging heat treatment. This
characteristic distinguishes the nickel maraging steels from all
other high strength alloys. Heretofore, it had been the
conventional wisdom that the heat treatments needed to improve the
mechanical and strength properties of steels were detrimental to
their magnetic properties. For example, quench hardened steel
alloys typically exhibit very low magnetic permeabilities and high
coercive forces, a combination of unfortunate magnetic properties
which materially decrease the sensitivity of such alloys to small
magnetic fields and diminish or negate their usefulness in torque
transducers such as are contemplated herein. This is demonstrably
not the case with the nickel maraging steels. In accordance with
the present invention it has been determined that nickel maraging
steels get magnetically softer following cold work and the aging
heat treatments to which they are conventionally subjected in order
to develop their extraordinary high strength characteristics. For
example, the coercive force of an 18% Ni maraging steel in fact
decreases when aged at 900.degree. F. for up to 10 hours. As a
result the maraging steels can be advantageously used in their aged
condition, i.e., in a condition where they exhibit maximum strength
characteristics and substantially the same or improved magnetic
characteristics.
Thus, the use of maraging steels as the magnetic material in a
magnetoelastic torque sensor, particularly as the shaft material in
a device whose torque is to be sensed, obviates virtually all of
the objections heretofore made to using the device shaft as the
magnetic member. The mechanical and strength properties of maraging
steels satisfy the mechanical properties requirements for most all
shaft applications while, at the same time, providing outstanding
magnetic properties for its role in the torque sensor. Aging of the
maraging steels provides the high strength and high hardness needed
for the mechanical application without loss of magnetic
permeability or increase in coercive force. Moreover, the
conventional manner of heat treating maraging steel, including the
initial solution anneal at temperatures in excess of 1500.degree.
F., relieves internal stresses due to mechanical working and most
stresses due to inhomogeneities and crystal orientation, thus
minimizing the amount of random magnetic anisotropy in a maraging
steel shaft. When such heat treatment is combined with the
creation, according to the present invention, of a pair of adjacent
bands endowed with intentionally instilled magnetic stress
anisotropy of a relatively large magnitude, e.g., by stressing the
shaft beyond its elastic limits with applied stresses of a
magnitude greater than the largest torque stresses anticipated
during normal usage of the shaft, the contribution to total
magnetic anisotropy of any random anisotropy in the shaft is indeed
negligible.
It will be appreciated that the advantage of the nickel maraging
steels in magnetoelastic torque transducers can be realized by
forming the shaft of the desired nickel maraging steel, by forming
a region of the shaft of the desired nickel maraging steel and
locating the annular bands within this region, or by surfacing with
a nickel maraging steel and a shaft formed of an alloy having
mechanical properties suitable for the intended function of the
shaft, i.e., applying over at least one complete circumferential
region of suitable axial extent of the shaft a surfacing alloy of
the desired nickel maraging steel and locating the annular bands
within this region. Inasmuch as magnetic permeability sensing in
accordance with the present invention is fundamentally a surface
phenomena, the surfacing process need apply a circumferential layer
of thickness not exceeding about 0.015 inches. The surfacing
process selected may advantageously be selected from among the many
known additive processes, e.g., electroplating, metal spraying,
sputtering, vacuum deposition, ion implanatation, and the like.
In order to demonstrate the outstanding qualities of the maraging
steels as the magnetic material in torque transducers of the
present invention and to compare the performance of maraging steels
with other high strength steels, a torque transducer was assembled
using a 12.7 mm diameter cylindrical shaft having formed thereon a
pair of axially spaced-apart bands endowed with helically
symmetrical LH and RH magnetic easy axes. The bands each had an
axial length of 12.7 mm and were separated by a 12.7 mm shaft
segment. They were formed by knurling using a 3/4-inch OD knurling
tool having 48 teeth around the circumference, each tooth oriented
at 30.degree. to the shaft axis. The characteristics of this
arrangement were sensed by positioning bobbins concentric with the
shaft and axially aligned with the bands, each bobbin having a
magnetizing and sensing coil mounted thereon. The magnetizing coils
were connected in series and driven by an alternating current
source having a 10 KHz frequency output and a 200 mA peak driving
current. The emf induced in each of the sensing coils was
separately rectified with the rectified outputs oppositely
connected to produce a difference signal which was displayed on a
voltage display instrument. Four shafts were employed, identical in
all respects except they were each formed of different materials.
The composition of each shaft is set forth in percent by weight
hereinbelow:
T-250: 18.5 Ni; 3.0 Mo; 1.4 Ti; 0.10 Al; less than 0.03 C; no
cobalt; balance Fe
SAE 9310: 0.08-0.13 C; 0.45-0.65 Mn; 3-3.5 Ni; 1-1.4 Cr; 0.08-0.15
Mo; balance Fe
416 SS: 11.5-13.5 Cr; 0.5 max Ni; 0.15 max C; 1.0 max Mn; 1.0 max
Si; balance Fe
AISI 1018: 0.15-0.20 C; 0.6-0.9 Mn; 0.04 max P; 0.05 max S; balance
Fe
In a first series of runs, the T-250 nickel maraging steel shaft
was used in the solution annealed, unaged condition as received
from Teledyne Vasco. Likewise, the other shafts were also used in
their as-purchased condition without further heat treatment. A
known torque loading was applied to each shaft under test and the
output voltage signal was recorded. The applied torque was
increased from zero up to 100 newton-meters (N-M). FIG. 5 is a
graph of applied torque versus output d.c. voltage for each shaft.
It is apparent that the sensitivity of the T-250 shaft in terms of
magnitude of output signal for a given torque loading was
significantly greater than for the other shaft materials tested. In
addition, the linearity of the output signal for the T-250 shaft
was extremely good over the entire torque range. The other shaft
materials appeared to be about equally insensitive, compared to the
T-250 shaft, to applied torque. None produced as linear a signal as
the T-250 shaft, although each produced a reasonably linear signal
over most of the torque range.
For the second series of runs, the T-250 nickel maraging steel
shaft was aged at about 900.degree. F. for 30 minutes to improve
the strength and hardness of the shaft. For consistency of testing,
the other shafts were heat treated in the same manner, after which
each shaft was subjected to an applied torque from zero to 100 N-M
and the output d.c. voltage recorded. FIG. 6 is a graph of applied
torque versus output d.c. voltage for each shaft after heat
treatment. It can be seen that once again the sensitivity of the
T-250 shaft far exceeded the sensitivity of the other shafts and
once again the T-250 output signal was linear over the entire
torque range. By comparison with FIG. 5 for the T-250 shaft in the
unaged condition it is apparent that aging measurably improved the
sensitivity of the shaft, indicating an enhancement of the magnetic
properties of the maraging steel with aging. By contrast, the
sensitivity of the SAE 9310 shaft did not appear to improve with
this heat treatment. Moreover, the linearity of the output signal
clearly degraded, particularly at higher applied torques. The
sensitivity of the AISI 1018 shaft significantly improved at low
applied torques but the improvement began to abate at about 40 N-M
and degraded thereafter. The linearity of the output signal for the
aged AISI 1018 shaft was very poor. For the 416 SS shaft, the
sensitivity at low applied torques improved with heat treatment but
significantly worsened at higher applied torques. The linearity of
the 416 SS output signal became worse with heat treatment. It is
noteworthy that notwithstanding the mixed response of the output
signal to applied torque, heat treatment adversely affected the
mechanical and strength properties of the SAE 9310, 416SS and AISI
1018 shafts. For example, following heat treatment, an applied
torque of only about 50 N-M exceeded the elastic limit of the AISI
1018 shaft and the shaft permanently twisted.
FIGS. 5 and 6 graphically illustrate the signal response to applied
torque using a relatively low, 10 kHz, a.c. excitation frequency.
It has been found that the output signal is directly proportional
to and increases approximately linearly with a.c. frequency. Tests
show that at 20 kHz, for example, a doubling of the output d.c.
voltage signal is obtained. Depending upon the circuitry employed,
a.c. frequencies in the range 1-1000 kHz can advantageously be used
to drive torque transducers of the present invention. Preferably,
frequencies of 10-30 kHz, just above the human audible range, are
used in order to avoid whistling. Most desirably, the frequency is
adjusted to about 20 kHz. Like its response to frequency, the
output d.c. signal also appears to be directly proportional to,
more specifically to vary sigmoidally with, the drive current
which, depending upon the frequency, can usefully be in the range
10-400 mA (peak). Generally, sufficient current is used to obtain a
good signal at the chosen frequency and, desirably, to adjust the
signal hysteresis to zero over the entire applied torque range.
It is interesting to note that the sensitivity of a nickel maraging
steel shaft is markedly better than the sensitivities reported by
workers employing non-magnetic shafts and adhesively affixing
amorphous ribbons thereto. From FIG. 6, it can be seen that
according to the present invention an aged T-250 nickel maraging
steel shaft transducer, having a shaft diameter of 12.7 mm,
produces an output d.c. signal of 0.9 volts at an applied torque of
60 N-M using an a.c. frequency of 10 kHz and an exciting current of
200 mA and employing exciting coils having 100 turns each and
sensing coils having 500 turns each, a sensitivity of 0.015 V/N-M.
By comparison, Sasada et al, in the paper "Noncontact Torque
Sensor", presented at the 11th Annual IEEE Industrial Electronics
Society Conference (Nov. 18-22, 1985) reports, for an amorphous
ribbon torque sensor, an output d.c. signal of 35 mV at an applied
torque of 10 N-M using an a.c. frequency of 20 kHz, an exciting
current of 120 mA, exciting coils having 220 turns each and sensing
coils having 80 turns each and a shaft diameter of 12 mm. Inasmuch
as sensitivity is directly proportional to a.c. frequency, exciting
current and number of turns on the exciting and sensing coils and
inversely proportional to the cube of the shaft diameter, the
Sasada et al sensitivity corrected to an equivalent basis as that
shown in FIG. 6 hereof is 0.007 V/N-M. In other words, the torque
transducer of the present invention is more than twice as sensitive
as the amorphous ribbon torque sensor of Sasada et al.
INDUSTRIAL APPLICABILITY
The unique and improved magnetoelastic torque transducers of the
present invention are broadly useful for the sensing and
measurement of torque in members of all types and sizes, whatever
may be the device or field of application in which the member
operates. It is universally accepted that torque is an absolutely
fundamental parameter in the control of systems having rotating
members. Sensing the instantaneous torque experienced by a rotating
member and generating an electrical current in response thereto
which bears a known relationship to the torque allows the early
diagnosis of incipient problems or the control, via microprocessor
or otherwise, of the engine, machine, motor, etc. which drives the
rotary member.
Applications for the torque transducers of the present invention
can be found in virtually every device having a rotating member.
There already is a demand for sensitive, responsive, and
inexpensive magnetic torque sensors for monitoring torque in
engines and power drives to improve overall performance and fuel
economy, control exhaust emissions and modulate transmission
ratios; in marine propulsion systems to detect and correct reduced
output from the propulsion machinery and the effects of hull
fouling and propeller damage; in helicopter turbines to avoid
overloading and to detect power loss caused, for example, by sand
or salt spray. There is also a demand for torque transducers such
as are provided in accordance with the present invention for
controlling heavy industrial machinery of all types, e.g., pulp
grinders for maintaining fiber quality, paper-making machines, and
the like, as well as for use in consumer home and commercial
appliances, e.g., food mixers and processors. In addition, the need
for small, inexpensive, sensitive, reliable torque sensors has been
noted in such diverse applications as machine tools, robotics,
information devices, industrial measuring instruments, weighing
systems of various kinds, electronic power assisted power steering,
and vehicular traction balancing.
One application for the magnetoelastic torque transducers of the
present invention which is particularly promising in view of the
potential contribution of these transducers to energy conservation,
environmental cleanliness and safety and because it directly
affects so many people and businesses is its use on internal
combustion engines and associated engine power drives. The torque
sensor of the present invention is capable of recovering the torque
signature of an engine over a wide enough bandwidth to discern
salient details of important torque contributing events at all
points between idle and the top operating speed of the engine.
Torque sensing in an accurate and cost effective manner enables
early diagnosis of incipient problems due to the functional
condition of the engine, helps to avoid unanticipated failures that
might limit the servicability of the vehicle at critical times and
improves and/or controls the performance and economy of the engine
and its power drive.
Primary power for the propulsion and other essential functions of
modern vehicles is obtained from the rotating output shaft of an
internal combustion engine. Regardless of the type of engine the
power actually delivered by this shaft to the vehicle is the
numerical product of only two parameters: rotational speed and
transmitted torque. Of the two, torque is the intensive parameter
since rotational speed is itself consequential to the internally
developed torque of the engine. It is the magnitude of available
torque that sets the limits on vehicle acceleration, its speed on
grade and other mobility and performance factors. The successful
use and enjoyment of the vehicle depends, ultimately, on the
ability of its engine to deliver the functionally required torque
throughout its operational range of speeds.
Except for the situation where a turbine engine is driving a
constant load, the torque transmitted through an engine output
shaft fluctuates rapidly. These fluctuations reflect both the
cyclic variations in the torque developed by the engine and
transient variations in the torque imposed by vehicle loads. In
piston engines, torque is developed by each cylinder only during
its power stroke. Multicylinder engines attain some continuity of
developed torque by the overlap of phased power strokes from each
cylinder. While cyclic variations in output torque are also reduced
thereby, and further reduced by the combined inertia of the
engine's internal moving parts, the strongly impulsive nature of
each cylinder's developed torque is still transmitted through the
output shaft. Cyclically stimulated torsional vibrations together
with the changing accelerations of linked reciprocating parts
contribute additional time varying torque components. The magnitude
and even the directional sense of this torque is further influenced
by variations in operational conditions of the vehicle, e.g.,
throttle settings, gear positions, load pick-up, road surface
inclination and roughness features.
Although the torque on the engine output shaft represents the
superposition of contributions from this multiplicity of sources,
many are strongly interdependent and their combination forms an
effective signature characterizing the engines's performance.
Salient features of this signature would clearly correlate with
specific engine events, e.g., cylinder firings. The absence of a
normal feature, its alteration or the development of new features
would be a reflection of a dysfunction. The nature and extent of
the abnormality would be symptomatic of specific engine or drive
line difficulties. While many engine problems are also detectable
by their symptomatic effects on overall performance and/or more
objectively measurable quantities (e.g., manifold pressure,
compression, noise signature, exhaust gas analysis), none are as
sensitively quantified as torque to the individual events which
together characterize proper engine function. Since torque is the
effective product of the engine, no measurements of indirectly
related parameters can so clearly identify the source of inadequate
production as can the measurement of torque itself. Conventional
methods of recovering torque data, whether by dynamometer or from
measurements of unloaded engine acceleration and deceleration by
procedures involving stepped changes in fuel flow and/or ignition
interruption, determine only average values and lack the detail
needed for clear diagnosis and control. Recovery and analysis of
the information contained in the torque signature of the engine
output shaft enables diagnosis of incipient problems, helps to
avoid unanticipated failures that might limit the servicability of
the vehicle at critical times and improves and/or controls the
performance and economy of the engine and its power drive. The key
to the problem is the recovery of enough torque information for a
meaningful analysis.
In a 12 cylinder, 4 stroke engine operating at 4000 rpm there are
400 power strokes and (at least) 1600 valving events (opening or
closings) every second. Turbine engines run with far smoother power
input but at speeds up to 500 revolutions per second. To be capable
of discriminating important details of these salient events, the
torque sensing system must have a reasonably flat frequency
response up to at least several times the maximum event rate, i.e.,
in the vicinity of 5 kHz. The frequency response must also extend
downward to zero Hz to faithfully capture the steady state torque
components imposed by the vehicle loads.
Although that full bandwidth is obviously desirable for maximum
utility as a diagnostic tool, the information contained in the low
frequency spectrum, up to 10 Hz, accurately describes the engine's
overall response to control (input) and load (output) changes. Not
only can variations in performance be objectively evaluated from
this information but it also has potentially prime utility in
another area, control of the engine and associated power drive.
A torque sensor having 5 kHz bandwidth capability cannot be
positioned arbitrarily. While torque is applied to the engine shaft
by contact forces at discrete locations, it is transmitted axially
by continuous stress distributions. Transient torque events are not
transmitted instanteously nor do they remain unaltered along the
shaft. The finite elasticity and inertia of real shaft materials
combine to limit the transmittable rate of change of torque. Steep
transients trigger oscillatory exchanges of elastic and kinetic
energy (stress waves) which travel with material and mode dependent
characteristic velocities along the shaft. The fidelity of the
transmitted torque is further reduced with distance from its source
by the accumulated dissipative effects of internal and external
friction. The sensor must therefore be located close enough to the
source(s) to avoid losing the desired torque information either by
attenuation or in background "noise" composed of complex
combinations of interfering and reflecting stress waves.
Important sensor requirements are that it be small, at least in the
dimension parallel to the shaft axis, that it be rugged and that it
be free from deteriorating effects of use or time such as wear,
corrosion or fatigue. The sensor should be amenable to performance
verification and calibration, especially in the event of repair or
replacement of parts of the torque sensing system, including the
engine shaft. It should have neglible impact on engine and drive
line manufacturability, operation and maintenance and, under no
circumstances should the failure of the torque sensor have any
contingent consequences which interfere with the otherwise normal
operation of the vehicle.
The context is clear, whether for engines, power drives or other
uses, a suitable torque sensor should be an unobtrusive device that
is difficult to abuse and is capable of reliably recovering much of
the torque information available on the torqued shaft. None of the
heretofore contemplated state of the art torque transducers can
meet these requirements. However, the magnetoelastic torque sensors
of the present invention appear eminently suitable in all respects
and will, for the first time, make inexpensive, reliable and
sensitive torque sensors available for commerical
implementation.
* * * * *